Characterization of a Major Secretory Protein in the Cane Toad (Bufo marinus) Choroid Plexus as an Amphibian Lipocalin-type Prostaglandin D Synthase

Here we report the enzymatic and ligand-binding properties of a major secretory protein in the choroid plexus of cane toad, Bufo marinus, whose protein is homologous with lipocalin-type prostaglandin (PG) D synthase (L-PGDS) and is recombinantly expressed in Xenopus A6 cells and Escherichia coli. The toad protein bound all-trans retinal, bile pigment, and thyroid hormones with high affinities (Kd = 0.17 to 2.00 μM). The toad protein also catalysed the L-PGDS activity, which was accelerated in the presence of GSH or DTT, similar to the mammalian enzyme. The Km value for PGH₂ (17 μM) of the toad protein was almost the same as that of rat L-PGDS (14 μM), whereas the turnover number (6 min⁻¹) was approximately 28 fold lower than that of rat L-PGDS. Site-directed mutagenesis based on a modeled structure of the toad protein revealed that Cys⁵⁹ and Thr⁶¹ residues were crucial for the PGDS activity. The quadruple Gly³⁹Ser/Ala⁷⁵Ser/Ser¹⁴⁰Thr/Phe¹⁴²Tyr mutant of the toad protein, resembling mouse L-PGDS, showed a 1.6 fold increase in the turnover number and a shift in the optimum pH for the PGDS activity from 9.0 to 8.5. Our results suggest that the toad protein is a prototype of L-PGDS with a highly functional ligand-binding pocket and yet with a primitive catalytic pocket.